In mobile radio systems such as cellular telephone, a large number of subscribers can be accommodated within a given frequency band by dividing a geographical area into smaller zones or cells and limiting the range of transmission to cell boundaries. The divided area arrangement permits reuse of channels within the system and greater utilization of the RF spectrum. A base station in each cell communicates with mobile units therein. All base stations are connected to a common control that determines call connections for the communication system. In this way, the same communication channels may be used concurrently in different cells.
When the quality of a connection between a base station and a mobile station is poor, the connection should be transferred to another channel within the cell or to another cell. In order to determine when such a transfer is required, the quality of signals received by the base station and the mobile unit must be monitored. If the monitored quality does not meet a predetermined standard, the common control is alerted and the connection is switched to another channel or handed off to another cell.
In analog communication channels, there are many ways to determine signal quality. These include measuring the relative strength of the signals received by a station and measuring the signal to noise ratio at the station receiver. U.S. Pat. No. 4,308,429 issued to Kai et al, Dec. 29, 1981, discloses a mobile telephone channel exchange system in which base station receivers detect carrier signal drop-off or carrier level which detected carrier levels are used to control channel transfers.
Where the channel carries digital signals, direct measurement of signal strength or signal to noise ratio may not be accurate due to the possible presence of interfering signals. Better alternatives for digital communication channels have been based on evaluation of the transmission error rate. In contrast to the direct measurements of signal used in analog channels, error rate evaluation generally requires some knowledge of the information being carried by the transmission channel. U.S. Pat. No. 4,829,519 issued to Scotton et al, May 9, 1989, for example, discloses an automatic cell transfer system with error rate assessment in which special messages are transmitted from a base station to a mobile receiver. The mobile receiver is equipped to monitor the bit or symbol error rate resulting from transmission of the special message by comparing the special message bits to a bit pattern stored in the mobile unit. Transfers between different base stations are made when prescribed error rate thresholds are crossed.
While special message error rate detection provides a satisfactory means for evaluating the quality of digital signals in a mobile communication channel, it requires periodic interruptions of information transmission over the channel. Consequently, the efficiency of data transmission and channel usage may be significantly reduced. It is an object of the invention to provide improved digital signal quality monitoring without reducing channel usage efficiency.